Information processing systems of the type that include light beam scanning, beam positioning, or beam tracking apparatus can benefit from the use of a controllably positionable light beam deflector. In high speed scanning or tracking systems, the beam deflecting mirror must be capable of rapidly and accurately directing an incident light beam to a desired location. Various mirror-type deflectors are known in the art. One type of deflecting mirror is electromagnetically driven and is commonly referred to as a galvanometer scanner. Beam deflectors of this type are expensive, complex, and can present hysteresis problems (see E.P. Grenda et al., "Closing the Loop on Galvo Scanners", Electro Optical Design, pp. 32-34, April, 1974).
Another type of beam deflector utilizes a mirror directly attached to a piezoelectric shear transducer that acts as a driver. The transducer driver is often referred to as a "bimorph" or a "bimorph bender". (See: J. J. Shaffer, et al., "Bender-Bimorph Scanner Analysis", Applied Optics, pp. 933-37, April, 1970; U.S. Pat. No. 3,544,201; U.S. Pat. No. 3,794,410; and U.S. Pat. No. 1,438,974). Bimorph scanners offer high performance, are simple in construction, and are low in cost. Because of these desirable characteristics, bimorph scanners have achieved general acceptance in the art. However, when evaluating a beam deflector, bandwidth becomes an important figure of merit. The practical limit for the bandwidth of a deflector may be taken as the fundamental resonant frequency f.sub.n. Very little angular movement can be achieved in a mechanical beam deflector beyond its fundamental resonant frequency. A disadvantage of beam deflectors of the type described above is their low fundamental resonant frequency and corresponding low bandwidth. High speed operation of galvanometer mirrors have been achieved, but only with use of small mirrors.
Hence, in applications wherein the mirror must be relatively large, such as 25 mm by 12.5 mm with the larger face being the rotating face, the galvanometer designs of the prior art are not suitable. In certain applications the mirror must also be very flat (1/10th wave or better) for optical reasons, which necessitates the use of a thicker mirror. Movement of such a large mirror at speeds above 10,000 Hz has heretofore been difficult if not impossible to accomplish.
Galvanometer-based mirror systems are thus generally considered too slow to provide the rapid corrections that are necessary for particularly critical optical systems. (See, for example, J. D. Zook, "Light beam deflector performance", Appl. Optics, 12, pp. 875-887, April, 1974, wherein the performance of electromagnetic galvanometers is shown to be governed by an upper limit determined by materials properties and the allowable heat rise. A high speed, small angle galvanometer is mentioned but its construction is not disclosed.) J. K. Lee, in "Piezoelectric optical beam scanners: analysis and construction", Appl. Optics, 18, pp. 454-459, February, 1979, describes a number of piezoelectric galvanometers but their resonant frequencies are on the order of 1 khz or less. Beiser, in "Laser Scanning Systems", Laser Applications, Vol. 2, Academic Press, pp. 53-159, 1974 describes a high speed shear mode piezoelectric scanner, but the cost of the piezoelectric element disclosed therein is too high for many applications. Acousto-optic (AO) or electro-optic (EO) deflectors are also known for beam deflection at relatively high speeds; however, they are costly and necessitate additional beam shaping optics.